The poorer the state of preservation of the ligament apparatus of a knee which is to be provided with a knee prosthesis, the greater inherent stability demanded of the prosthesis. If it is a ligament apparatus of reduced stability, it may be necessary to use a prosthesis type in which the relative movement between femoral component and tibial component is limited by a stabilizing intermediate part to the flexion movement about a horizontal transverse axis and to a rotation movement about an axis substantially parallel to the tibia (DE-C-2 744 710). For this, it is required that the ligament apparatus can at least still control the rotation movements. If this ability too has been lost, the rotation movement must be excluded and the degree of freedom of the prosthesis must be limited to the flexion movement. Different prosthesis structures are normally used for these two cases of application. If the decision whether to choose a rotation-free or rotationally fixed prosthesis is to be made only during the operation, a prosthesis system can be used whose components are partly exchangeable (EP-B-539 654), with one matching femoral component and different tibial components being provided. Finally, it has also already been proposed (DE-A-2 636 816) to use for both cases a rotation-free prosthesis in which a fixing device can be arranged which excludes the rotation movement if so desired. This is intended to be done by drilling a hole through the bone and the rotation bearing after implantation and then introducing a fixing pin into the hole to prevent turning of the rotation bearing. This is obviously impracticable. Instead, a fixing pin can also be guided exclusively inside the prosthesis through a rotation axial bearing and prevent its turning. It has not been disclosed whether this proposal has ever been adopted in practice. In fact, there are a great many practical difficulties against it. One factor may have been that, compared to a prosthesis which exclusively includes a flexion hinge, it is relatively expensive to use a prosthesis additionally provided with a rotation bearing and then to lock this.